1. Field of the Invention
The present invention relates to the epitaxial growth of hetero-atomic single-crystal layers. The present invention more specifically relates to the forming of such layers on a semiconductor material of different crystallographic parameters.
2. Discussion of the Related Art
The forming of silicon-germanium layers, SiGe, on a single-crystal silicon substrate will be considered hereafter as an example, and the case where a very thin strained silicon layer is desired to be formed on the SiGe layer will more specifically be considered.
In a given crystallographic system, in which the elementary lattice has a determined dimension, the electronic characteristics, and especially the mobility of the carriers (electrons/holes), are determined. It has been shown that it could be desirable to modify the lattice parameters to optimize the electronic characteristics. In particular, in the case of silicon, it has been shown that, if the dimension of the elementary lattice was increased (the value of the lattice parameter), this would enable forming of elementary components, of which a critical dimension (for example, the channel length or the base thickness) could be reduced.
To obtain silicon with an increased elementary lattice (strained silicon), it has been provided to form a silicon layer on a crystal layer having a lattice larger than the normal lattice of single-crystal silicon. In particular, it has been provided to deposit a very thin layer (a few atomic thicknesses, for example, from 5 to 20 nm) of silicon on a silicon-germanium layer of selected composition to have a determined lattice parameter.
FIG. 1 is a partial simplified cross-section view illustrating the forming, on a single-crystal silicon substrate 1, of a silicon-germanium layer 2 which will be called the “pseudo-substrate” and of a silicon layer 3. Upon epitaxial deposition of pseudo-substrate 2, the latter grows, on a few first atomic thicknesses, with a same lattice parameter a1 as that of substrate 1. Then, the presence of germanium causes a progressive deformation of the crystal lattice, the lattice parameter of which varies from value a1 to a value a2. Final value a2 depends on the proportion of introduced germanium, possibly increasing along the deposition. Upon subsequent epitaxial deposition of silicon layer 3, said layer starts its growth with a strained lattice parameter equal to underlying parameter a2. Electronic components may then be formed so that at least one of their portions, for example, a MOS transistor channel region or a bipolar transistor base area, is formed in layer 3.
It is thus desired to obtain a defect-free layer 3. “Defect-free” here means comprising a surface density of defects on the order of that of standard silicon substrates, of approximately 1/cm2. The defects of layer 3 are mainly the consequence of the defects of pseudo-substrate 2. Now, current methods of epitaxial forming of silicon-germanium on silicon result in SiGe layers exhibiting too high a defect density, over 104/cm2.